Certain injuries or conditions within the human body often require that bodily tissue be firmly affixed to an adjacent bone structure to secure the tissue in a desired position, for instance, where a ruptured tendon must be reattached to the corresponding bone. To this end, suture anchors have been developed, providing an anchor in the bone to which a suture may be secured, wherein the suture is then used to secure the adjacent bodily tissue. Suture anchors generally are inserted into a bore formed in the bone. There are several types of suture anchors in the prior art, including a barb-type anchor, a corkscrew type anchor, an insert type anchor with offset reorienting suture bore, and a wedge-type anchor. Of these types of suture anchors, the wedge-type anchor has shown good promise.
In practice, wedge-type anchors with attached sutures are first introduced with an insertion tool into a bore drilled into the bone at a location where the bodily tissue must be affixed. The suture anchor generally includes a first gripping portion, such as a sharp edge or a point, and is designed to rotate as it enters the bore or is in the process of being withdrawn therefrom. As the suture anchor rotates, the first gripping portion bites or penetrates the wall of the bore and causes further rotation of the anchor. At the opposite end of the suture anchor, a second gripping portion is typically provided. The second gripping portion penetrates into the wall of the bore generally opposite the first gripping portion as the suture anchor is rotated, thereby wedging the suture anchor within the bore. When the suture anchor has rotated about the first gripping portion as much as the second gripping portion will allow with respect to the bone structure surrounding the bore, the suture anchor is in its seated position where it is anchored in the bore and ready to have adjacent bodily tissue attached thereto.
However, while suture anchors are theoretically designed to work with all bones, the structure of bone can vary greatly depending on the area of the body in which the suture anchor is required. Bones generally include a number of trabeculae disposed throughout. The spacing of the trabeculae within the intermedulary canal of the bone is often a good indicator of the density of the bone. Cortical bone is solid bone without visible interstitial spaces and is typical of the midshafts, or diaphyseal regions, of long bones. Metaphyseal and epiphyseal bone, which is the bone around the joints at the opposing ends of the long bones, has a variable amount of cortical shells with a deeper trabecular structure, wherein the amount of cortical shells may vary greatly. Dense bone typically has small and closely spaced trabeculac, resulting in a hard and strong bone. In comparison, less dense or osteoporotic bone has larger and more widely spaced trabeculae, typically resulting in a softer and generally weaker bone. This less dense bone generally comprises a cancellous bone region about the intermedulary canal. A typical bone structure thus typically includes a cortical layer atop cancellous layer where the proportion of both types of bone may vary. Since the structure of bone may vary significantly from one area of the body to another, the specific performance characteristics required of a suture anchor also vary accordingly. Therefore, suture anchors must be able to function as intended when used in all types of bone structure. Where suture anchors are implanted into dense bone structures, well defined strong gripping edges are required for the suture anchor to securely engage the bore in the bone. However, where the suture anchor is implanted into less dense bone, a large surface area engaging the bone is preferred in order to distribute and reduce the stresses on the relatively weak bone.
Examples of wedge-type suture anchors are found in U.S. Pat. Nos. 5,540,718, 5,626,612, 5,782,863, and 5,879,372 to Bartlett which disclose wedge-type suture anchors based on variants of cone, double cone, and combination cone and cylinder configurations. Additionally, U.S. Pat. No. 5,683,418 to Luscombe et al. assigned to Mitek Surgical Products, Inc., also the assignee of the present invention, discloses wedge-type suture anchors generally varying in shape from triangle to a quadrilateral. Thus, U.S. Pat. Nos. 5,540,718, 5,626,612, 5,782,863, and 5,879,372 to Bartlett and U.S. Pat. No. 5,683,418 to Luscombe et al. are incorporated in their entirety herein by reference.
The Luscombe et al. '418 patent discloses suture anchors which are angularly shaped, generally approximating a triangle to a quadrilateral and having straight sides. The intersections of major surfaces defining the perimeter of the suture anchor therefore define sharp transitions. Thus, the device disclosed by the '418 patent is generally more useful in dense bone structures where the sharp gripping edges more effectively bite into the bore in the dense bone. However, with the sharp gripping edges biting into the bone, the '418 device generally experiences less rotation in both dense and less dense bone than is desirable to properly seat the anchor. Rotation is further inhibited by the long straight surfaces defining the device.
A possible additional limitation of the '418 device is that the insertion tool used for implanting the suture anchor in the bore is formed from a material having a frangible portion at or near the interface with the anchor which breaks or separates from the anchor after implantation thereof. In the alternative, the '418 device uses a metallic insertion tool which engages a bore in the anchor in a friction fit. Both insertion tools may experience problems in that the tool with the frangible portion may not provide enough strength for manipulating the anchor within the bore, while the friction fit tool may be difficult to separate from the anchor after implantation thereof. The Bartlett '718, '612, '863, and '372 patents disclose wedge-type suture anchors based on variants of cone, double cone, and combination cone and cylinder configurations. These devices use the tip of the cone, or a truncation thereof, to form the edge which first bites into the bone and then provides the point about which the anchor rotates. However, with the reduced cross-sectional area at or near the tip of the cone, this main biting edge may be weaker and prone to breaking in high stress situations, such as where the anchor is being implanted in a dense bone structure. In addition, wedge-type suture anchors generally use a suture, engaged with the anchor through a bore therein, to assist in rotating and seating the anchor within the bone. The wedge-type suture anchors generally have the bore located between the main biting edge and the bore in which the insertion tool is inserted, thereby providing a small moment arm for rotating the suture anchor using the suture. Thus, the suture may be less helpful in assisting the suture anchor to rotate within the bore in the bone, especially in a dense bone structure where the anchor is more difficult to rotate. On the other hand, the Bartlett devices may be more effective in less dense bone where the main biting edge does not experience as high a stress during the seating process for the anchor. Further, the anchor may be able to rotate to a greater extent within the bore in a less dense bone, thus allowing the suture to be more effective in rotating and seating the anchor. Accordingly, since the Bartlett devices are able to rotate more extensively in less dense bone, the anchor can be more effectively seated, thus providing the greater surface area which is desirable for a suture anchor used in less dense bone.
Thus, there exists a need for a suture anchor which will effectively anchor a suture within a bore in a bone while also preferably functioning equally in bones of differing densities, from hard dense bone to softer, less dense bone.